This invention relates to an apparatus and method for analyzing charged particles.
Measurement of electron yield is a known technique which is used to study the response of material surfaces to interactions with beams of high energy photons such as X-rays. Exposing a surface of a material to X-rays of varying energies, and measuring the number and energy of electrons emitted from the surface, makes it possible to analyze the chemical characteristics of the surface in detail.
A known method of measuring electron yield involves placing a sample in a high vacuum environment, exposing the sample to a beam of X-rays, and detecting a total electron yield current at a collection plate. The sample is held at a negative voltage relative to the collection plate to promote the drift of electrons from the sample to the collection plate. A variation of this method is to use a gas filled ion chamber, where the electron yield is measured indirectly as part of a total current from the sample. There is no overall gain employed in either of these methods, and the currents measured should correspond exactly to the number of electrons which leave the sample. These currents are small and are consequently prone to distortion by xe2x80x98pick-upxe2x80x99 and xe2x80x98leakagexe2x80x99 in the local environment. A method of obtaining noise free or close to noise free gain of the current of electrons emitted from the sample surface would significantly enhance the sensitivity of the electron yield measurement.
One approach to providing an increased sensitivity of electron yield has been to hold the sample in a high vacuum environment, and use an electron multiplier (such as a microchannel plate). However, useful samples are not ideal for high vacuum systems and a method of measurement which could operate at atmospheric pressure would be much more useful. The possibility of obtaining chemical reactions on the surface of the sample must also be considered. To this end, an alternative approach to measuring electron yield has been devised, wherein a sample and detector are surrounded with a gas mixture. The gas mixture is chosen such that, under the appropriate conditions, an electron emitted from the sample surface will ionize gas molecules, thereby forming an electron cloud in the gas. The number of electrons in the cloud will be equal, within statistical limits, to the energy of the initial electron divided by the ionization energy of the gas.
The electron cloud is subsequently accelerated along a radial field radiating from a wire anode. The electrons accelerate until they acquire sufficient energy to cause fisher ionization of the gas molecules, thereby creating more electrons, which are accelerated and ionize further electrons, in what is known as an avalanche effect. The strength of the field at the wire is limited so that the number of electrons produced by the ionization is proportional to the number of electrons in the cloud formed at the surface of the sample.
Since only a single wire is used to detect electrons, the detection system is only capable of measuring the total number of electrons arriving at the wire in any given time interval, and does not provide any spatial information. More specifically, the wire detection system does not allow a user to determine from what position on the surface of a sample electrons have been emitted.
It is an object of the present invention to overcome or substantially mitigate the above disadvantages.
According to the invention there is provided a charged particle analyzer comprising a source of charged particles and a charged particle detector spaced from the source and immersed with the source in an ionizable gas, wherein the detector comprises at least one pair of electrodes which are spaced apart by a distance that is substantially less than the spacing between the source and detector, the electrodes of the pair are maintained at different potentials, and the source is maintained at a potential different from the potentials of the electrodes, the potentials being selected such that charged particles emitted by the source are attracted from the source towards each of the pair of electrodes, and such that charged particles adjacent the detector are accelerated to energies sufficient to ionize the gas.
If the charged particles are negatively charged, the source is held at a potential which is more negative than the potentials of both the electrodes of the pair.
Preferably, the source is a sample and means are provided for exposing the sample to a beam of radiation the energy of which is sufficient to cause charged particles to be emitted from the sample. The exposing means may comprise an X-ray source. The sample may define a surface which is substantially planar and the beam may be directed towards the sample in a direction inclined to a normal to the sample surface. The beam may be directed at a glancing angle relative to the sample surface.
Suitably, the energy beam is monoenergetic and the energy of the beam is varied with time to provide analysis over a range of energies.
Suitably, the energy of the beam is non-uniform such that the incident energy varies across an area of the sample irradiated by the beam, and the electrodes are positioned to receive charged particles form different portions of that area, the incident energy varying between said portions.
The electrodes are preferably defined by parallel strips of conductive material mounted on an semiconducting substrate. The electrodes of the pair may be located so as to be substantially equidistant from the source.
The invention also provides a method for analyzing charged particles emitted by a source, wherein the source and a charged particle detector are immersed in an ionizable gas, the detector being provided with at least one pair of electrodes, characterized in that the electrodes of the pair are spaced apart by a distance substantially less than the distance between the source and the detector, different potentials are applied to the electrodes, and the source is maintained at a potential different from the potentials of the electrodes, the potentials being selected such that charged particles emitted by the source are attracted from the source towards each of the pair of electrodes, and such that charged particles adjacent the detector are accelerated to energies sufficient to ionize the gas.